Editing Hubble Deep Field (section)

==Observations==
[[File:Hubble Deep Field observing geometry.svg|thumb|left|The HDF was in Hubble's northern Continuous Viewing Zone, as shown by this diagram.]]
[[File:Hubble Ultra Deep Field diagram.jpg|thumb|Diagram illustrating comparative sampling distance of the HDF and the 2004 [[Hubble Ultra-Deep Field]]]]

Once a field was selected, an observing strategy was developed. An important decision was to determine which [[filter (optics)|filter]]s the observations would use; WFPC2 is equipped with 48 filters, including [[narrowband]] filters isolating particular [[emission line]]s of [[astrophysical]] interest, and [[broadband]] filters useful for the study of the colors of stars and galaxies. The choice of filters to be used for the HDF depended on the [[throughput]] of each filter—the total proportion of light that it allows through—and the spectral coverage available. Filters with [[bandpass]]es overlapping as little as possible were desirable.<ref name="Williams1996" />

In the end, four broadband filters were chosen, centred at [[wavelength]]s of 300 [[Nanometre|nm]] (near-[[ultraviolet]]), 450&nbsp;nm (blue light), 606&nbsp;nm (red light) and 814&nbsp;nm (near-[[infrared]]). Because the [[quantum efficiency]] of Hubble's detectors at 300&nbsp;nm wavelength is quite low, the noise in observations at this wavelength is primarily due to [[charge-coupled device|CCD]] noise rather than sky background; thus, these observations could be conducted at times when high background noise would have harmed the efficiency of observations in other passbands.<ref name="Williams1996" />

Between December 18 and 28, 1995—during which time Hubble orbited the Earth about 150 times—342 images of the target area in the chosen filters were taken. The total exposure times at each wavelength were 42.7 hours (300&nbsp;nm), 33.5 hours (450&nbsp;nm), 30.3 hours (606&nbsp;nm) and 34.3 hours (814&nbsp;nm), divided into 342 individual exposures to prevent significant damage to individual images by [[cosmic ray]]s, which cause bright streaks to appear when they strike CCD detectors. A further 10 Hubble orbits were used to make short exposures of flanking fields to aid follow-up observations by other instruments.<ref name="Williams1996" />